This invention relates to new and useful fluoropolymer composites comprising coated substrates. More particularly, the invention relates to a new fluoroelastomer/fluoroplastic matrix useful as a coating in the manufacture of reinforced woven composites which are flexible, exhibit good matrix integrity, and possess good adhesion or bonding of the coating matrix to the substrate. The invention includes composites which also have extraordinary chemical resistance, particularly at elevated temperatures and in humid environments. The invention further relates to a method of making such composites whereby the desirable high temperature, chemical inertness of fluoroplastic materials is combined with the desirable mechanical properties of fluoroelastomers in such a way as to maintain a desirable fabric-like flexibility.
Perhaps the most well-known subclass of fluoropolymers are substances called "perfluoroplastics" which are generally recognized to have excellent electrical characteristics and physical properties, such as a low coefficient of friction, a low surface free energy (i.e., non-wetting to many organic fluids), and a very high degree of hydrophobicity. Fluoroplastics, and particularly perfluoroplastics (i.e., those fluoroplastics which do not contain hydrogen), such as polytetrafluoroethylene (PTFE), fluoro (ethylene-propylene) copolymer (FEP) and copolymers of tetrafluoroethylene and perfluoro-propyl vinyl ether (PFA), are resistant to a wide range of chemicals, even at elevated temperatures, making them particularly useful in a variety of industrial and domestic applications. However, due to the partially crystalline nature of these fluoroplastics, they exhibit a degree of stiffness or lack of compliance which is detrimental to the utilization of these desirable properties. This shortcoming is particularly noticeable and objectionable in a reinforced composite where some degree of flexibility, elasticity, and/or conformability is necessary.
The broad class of fluoropolymers also includes substances called "fluoroelastomers" which are not only elastomeric, but also possess, although to a lesser degree, the aforementioned physical and electrical properties of a fluoroplastic. Fluoroelastomers, including perfluoroelastomers, have the low flex modulus and conformability which fluoroplastics lack. The hydrogen-containing fluoroelastomers, however, do not maintain other advantageous physical properties associated with fluoropolymers over as broad a temperature range, or at as high a level, as do the perfluoroplastics. In other words, perfluoroplastics simply perform better over a wider temperature range. Moreover, the fluoroelastomers which contain hydrogen (i.e., which are partially fluorinated) generally degrade rapidly at higher temperatures resulting not only in the loss of physical integrity but also in the formation of hydrofluoric acid. Hydrofluoric acid is, of course, highly corrosive to most materials, including those normally used as reinforcing substrates for textile composites, and particularly to fiberglass substrates. For this reason, hydrogen-containing fluoroelastomer based composites presently used in high temperature environments require relatively frequent replacement. Notwithstanding these drawbacks, fluoroelastomers containing hydrogen are considered excellent candidates for use in a variety of commercial applications requiring a lower flex modulus than that possessed by the stiffer fluoroplastics.
In this regard, attempts have been made to employ reinforced fluoroelastomer composites where good thermochemical, as well as mechanical properties, i.e. low modulus, are required at higher temperatures. One such application is in high temperature expansion joints which connect large duct sections in applications such as power plant systems. These ducts have in the past been joined at their section ends by metal bellows which, while basically chemically and thermally sound, provide minimal thermo-mechanical shock resistance under normal operating conditions, which can involve temperatures up to 550.degree. F., or even 650.degree. F. In an effort to improve the mechanical properties of metal expansion joints, the flexibility of an industrial fabric is desired, and fabric composites coated with fluoroelastomer based rubber compounds have been used.
These fabric composites have used various reinforcement materials, including fiberglass fabric, coated with a matrix containing a fluoroelastomer composition based on copolymers of hexafluoropropylene (HFP) and vinylidene fluoride (VF.sub.2) or terpolymers including HFP, VF.sub.2 and tetrafluoroethylene (TFE). The fluoroelastomer materials used all contain at least some hydrogen and, as such, are susceptible to the shortcomings associated with hydrofluoric acid elimination. Moreover, in order for the prior art fluoropolymer composites to be useful in high temperature, chemically corrosive applications, they customarily incorporate a relatively thick matrix of the fluoroelastomer based rubber, thereby increasing their stiffness and potentially aggravating problems deriving from hydrofluoric acid formation and thermal embrittlement. In an effort to avoid these problems, composites using hydrogen-containing fluoroelastomer compounds are being reinforced with acid resistant alloys such as INCONEL, or high temperature synthetics, such as NOMEX and KEVLAR. None of these composites, however, offer the desired combination of thermal and chemical resistance with acceptable matrix integrity.
Even where chemically insusceptible substrates, such as PTFE, have been coated with fluoropolymers, such as in Westley, U.S. Pat. No. 3,513,064, the resulting composites could only be achieved by selecting specific coating materials as limited by processing conditions, such that the composites possessed properties permitting use only in certain narrow applications.
In the hope of achieving an improved balance of fluoropolymer properties, prior attempts have been made to combine the respective good properties of fluoroplastic and fluoroelastomer materials in the manufacture of coated fabric. But these attempts have produced blends which either suffer the combined disadvantageous properties of the components or exhibit diminished good properties, particularly at higher temperatures, for example above about 500.degree. F. A typical example of these prior attempts is found in U.S. Pat. No. 3,019,206 to Robb.
While perfluoropolymers, whether thermoplastic or elastomeric, possess excellent thermal and chemical stability, it is difficult to form durable bonds between them and other materials due to their low surface free energy and chemical inertness. This difficulty is conventionally obviated by providing roughened surfaces to promote mechanical bonding, such as employing inorganic fillers or abraded surfaces. Specific surface treatments, such as those based upon chemical etching, may also be employed. But none of these known techniques results in bonding which is particularly strong or durable under environmental stresses, such as ultraviolet or thermally induced oxidation.
Accordingly, it is an object of this invention to provide a fluoropolymer composite comprising a substrate coated with a fluoroelastomer/fluoroplastic matrix. The invention composite is flexible, exhibits good matrix cohesion, and possesses excellent adhesion of the matrix to the material acting as the reinforcement or substrate, while maintaining the low stiffness associated with a fluoroelastomer combined with, where desired, the superior high temperature performance of a fluoroplastic.
It is also an object of this invention to provide a fluoropolymer composite which is relatively light, but strong, and which is both chemically and thermally superior, particularly at elevated temperatures and under humid conditions, while ameliorating the polymer degradation problems that have heretofore arisen in the use of composites having a coating matrix based upon a hydrogen-containing fluoroelastomer.
It is a further object of this invention to provide a fluoropolymer composite having outstanding thermochemical properties for use as chemical liners, expansion joints, and life safety devices, such as escape hoods, escape chutes and chemically protective clothing.
It is yet another object of this invention to provide a composite having the combined advantages of perfluoroplastics and fluoroelastomers which can be used to make excellent plied constructions, including multiple biased-plied composites, as well as composites having a single coated face.